The present invention relates to a high-frequency power supply apparatus for a plasma generation apparatus, and particularly to a high-frequency power supply apparatus for a plasma generation apparatus in which the output of the power supply apparatus can be made intermittent so as to make the average output of the power supply under control.
First, a background art will be described on a plasma processing apparatus called an ECR (Electronic Cyclotron Resonance) system by way of example.
In the plasma processing apparatus of the ECR system, a microwave is introduced into a vacuum vessel to which a magnetic field is applied from the outside so as to generate plasma. Electrons make cyclotron motion due to the magnetic field. If the frequency of the cyclotron motion and the frequency of the microwave are made resonant with each other, it is possible to generate plasma efficiently. In order to accelerate ions entering a sample (or wafer), a high-frequency voltage is applied to the sample. As a gas for generating plasma, a halogen gas such as chlorine or fluorine is used.
In such a plasma processing apparatus, the output of the high-frequency power supply apparatus to be applied to the sample (wafer) is subject to ON/OFF control so that the output of the high-frequency power supply apparatus is controlled for the purpose of improvement in accuracy of finishing. By such control, the selection ratio of silicon (Si) which is a substance to be etched to a foundation oxidation film can be made high and the aspect-ratio dependence can be reduced.
Further, the output of the high-frequency power supply apparatus to be applied to a wafer is subject to ON/OFF control so that it becomes possible to reduce ablation of a sample table without lowering the wafer processing speed. Accordingly, the frequency of maintenance for sample table exchange can be reduced and the throughput of the apparatus can be improved.
FIG. 6 is a drawing for explaining the relation between the average output and the peak output value. In FIG. 6, the output of the high-frequency power supply apparatus has a waveform 141. Further, the high-frequency power supply apparatus has an ON-time 144 and an OFF-time 145 for a pulse output, and has a peak output value 142 and an average output 143. For example, when polysilicon is subject to etching, the electric power ranging from 10 W to 100 W is supplied as an average output.
Further, the duty ratio (ON-time ratio), which is a typical one for use in the ON-OFF control, is about 20%, and the average output of the high-frequency power supply apparatus is about 40 W when the peak output value of the high-frequency power supply apparatus is 200 W.
FIG. 7 is a diagram showing a high-frequency power supply apparatus for a plasma generation apparatus in the background art. In FIG. 7, there are provided: a controller microcomputer 1 for operating the high-frequency power supply apparatus; a high-frequency power supply apparatus (hereinafter simply referred to as xe2x80x9chigh-frequency power supplyxe2x80x9d) 20 for a plasma generation apparatus; a peak value setting signal 21 for setting a peak value of the output of the high-frequency power supply; a repetition frequency setting signal 36 for setting the repetition frequency of ON-OFF control; an ON-time interval setting signal 37 for setting the ON-time interval when ON-OFF control is carried out; an output controller 22 for generating an operation signal 23 for operating the output peak on the basis of the peak value setting signal 21 and a fed-back peak value detection signal 30; a modulator 24 for generating a high-frequency output 25 which is made intermittent on the basis of the operation signal 23 and a modulation reference signal 31 which will be described later; a peak value detector 29 for detecting the above-mentioned intermittent high-frequency output at the timing of a pulse reference signal 38; and a pulse generator 35 for generating the modulation reference signal 31 and the pulse reference signal 38 on the basis of a repetition frequency setting signal and the ON-time interval setting signal 37 supplied from the controller microcomputer 1. The pulse generator 35 generates a rectangular wave, with a predetermined duty ratio, on the basis of the signals indicating the above-mentioned repetition frequency and the ON-time interval. The pulse generator 35 outputs the rectangular wave as the pulse reference signal 38, and further outputs, as the modulation reference signal 31, a signal obtained by multiplying the pulse reference signal 38 by a high-frequency reference signal (output of an oscillator).
First, the peak value setting signal 21 is set by the controller microcomputer 1. On the basis of the peak value setting signal 21 and the peak value signal 30 detected by the peak value detector 29, the output controller 22 outputs the operation signal 23 for controlling the peak value of the output of the high-frequency power supply 20. The modulator 24 generates intermittent high-frequency output 25 on the basis of the operation signal 23 and the modulation reference signal 31 generated in the pulse generator 35. The peak value detector 29 detects the high-frequency output which is intermittent at the timing at which the pulse reference signal 38 is supplied. By this arrangement, the detector 29 can detect the peak value of the high-frequency output (the peak output value in the ON-period of the intermittent high-frequency output). The pulse generator 35 generates a rectangular wave, with a predetermined duty ratio, on the basis of the signal 36 indicating the repetition frequency and the signal 37 indicating the ON-time interval. The rectangular wave is supplied, as the pulse reference signal 38, to the peak value detector 29 so as to be used as timing information for detecting the peak value. Further, the pulse generator 35 generates, as the modulation reference signal 31, a signal which is intermittent at the same timing as the pulse reference signal 38, and supplies this signal 31 to the modulator 24. On the basis of the peak value operation signal 23 and the modulation reference signal 31, the modulator 24 generates the high-frequency output 25 having a peak value indicated by the peak value operation signal and having duty ratio and the repetition frequency indicated by the modulation reference signal.
The average value of the above-mentioned intermittent high-frequency output can be calculated by multiplying the peak value detected by the peak value detector by the duty ratio (ON-period/ON-period+OFF-period). That is, if the peak value detected by the peak value detector, the repetition frequency setting signal and the ON-period setting signal are monitored, a high-frequency output at a desired average value can be supplied to the load.
However, the real output waveform (pulse waveform) of the high-frequency output subjected to intermittent control may generate distortion in the rectangular wave. FIGS. 8A and 8B are views showing the waveforms of the intermittent high-frequency outputs. Particularly, FIG. 8A shows the intermittent waveform of the high-frequency output, and FIG. 8B is an enlarged view of a part of FIG. 8A. As shown in FIGS. 8A and 8B, the waveform of the output power may fluctuate in the leading/trailing edge due to individual differences in the response property of the above-mentioned modulator, or due to individual differences between the parts. Accordingly, if the control is performed only through monitoring of the above-mentioned peak value, it is impossible to accurately calculate the real output value (the average value) in the above-mentioned method.
Incidentally, the fluctuation in the average output which is supplied to the load may give a large influence to the etching property given to wafers in the semiconductor finishing process. Further, even in the case where another high-frequency power supply with the same specification is used simultaneously, a difference is generated in the etching rate in the surface of the wafers, so that the process properties may fluctuate.
The invention has been achieved in consideration of such problems, and the present invention provides a high-frequency power supply for a plasma generation apparatus, in which the output accuracy in the intermittent high-frequency output can be improved.
In order to obviate these problems, the present invention adopts the following means.
A high-frequency power supply for a plasma generation apparatus provided with a modulator for generating an intermittent high-frequency output on the basis of a modulation reference signal and a peak value setting signal, the high-frequency power supply having: a first control loop including a peak value detector for detecting a peak value of the intermittent high-frequency output, and an output controller for comparing the peak value of the intermittent high-frequency output detected by the peak value detector with a preset peak value of the high-frequency output so that the output controller controls the peak value of the high-frequency output to be the preset peak value; and a second control loop including a monitor for detecting an average value of the intermittent high-frequency output, an arithmetic operation means for calculating an average value of the intermittent high-frequency output on the basis of the preset peak setting value and a preset duty ratio setting value, and a pulse generator for generating a modulation reference signal for controlling the modulator on the basis of the average value of the high-frequency output detected by the monitor and the average value of the high-frequency output calculated by the arithmetic operation means.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings